a) Field of the Invention
The invention is directed to the field of electrical engineering and electronics. Objects in which the invention may possibly and advantageously be applied are components based on surface acoustic waves such as bandpass filters with a relative bandwidth of up to several percent and oscillators.
b) Description of the Related Art
Surface acoustic wave components are known in which two single-phase unidirectional transducers which are composed of base elements comprising a pair of fingers and a reflection finger are arranged on a piezoelectric substrate, wherein the two fingers of the pair of fingers have opposite polarities and are at a distance of one quarter wavelength from one another. One of the fingers of the pair of fingers and the reflection fingers are connected with ground potential, while the second finger of the pair of fingers is connected to a potential not equal to the ground potential (C. S. Hartmann and B. P. Abbott, 1989 IEEE Ultrasonics Symposium Proceedings, pages 79-89 [1]; E. M. Garber, D. S. Yip and D. K. Henderson, 1994 IEEE Ultrasonics Symposium Proceedings, pages 7-12 [2]). The single-phase unidirectional transducer is designated as such or, in abbreviated form, as SPUDT in English-language technical literature.
In a special construction (P. Ventura, M. Solal, P. Dufilie, J. M. Hode and F. Roux, 1994 IEEE Ultrasonics Symposium Proceedings, pages 1-6 [3]), as also in [1] and [2], the distance from the center of the reflection finger to the center of the nearest finger of the respective pair of fingers is the same in all base elements of both single-phase unidirectional transducers. The distance of the effective location of wave excitation from that of reflection, referred to as the excitation center and reflection center, respectively, is 3/8 of the wavelength. The reflection centers are located in the center of the reflection fingers, while the excitation centers are positioned close to the center of those fingers of the pairs of fingers connected to a potential other than ground potential.
Due to the distance of one quarter wavelength between the two fingers of a pair of fingers, the pairs of fingers are non-reflecting, but excite surface acoustic waves, namely, effectively at the site of the excitation center. Reflections take place exclusively at the reflection fingers. Because of the distance of 3/8 of the wavelength between the excitation center and reflection center, there is a phase difference of 3.pi./2 between the directly emitted wave and the reflected wave. In this respect, the phase displacement of .pi./2 is summed due to the reflection at a finger with front edge and rear edge and therefore gives a total phase displacement of 2.pi., wherein the reflection factor at an edge is assumed to be real. Consequently, directly emitted waves and reflected waves overlap constructively in the direction pointing from the reflection finger to the pair of fingers of one and the same base element. Typically, a single-phase unidirectional transducer is formed of a periodic arrangement of base elements with a period length equal to a wavelength. Therefore, the distance of an excitation center (approximately the center of the finger of a pair of fingers not connected to ground potential) from the next reflection finger in the opposite direction is 5/8 of the wavelength, which gives a phase displacement of 3.pi. between the wave emitted in the opposite direction and the wave reflected in the opposite direction. In other words, with respect to the directly emitted waves, the reflected waves are real. Consequently, in the direction pointing from the pair of fingers to the reflection finger of one and the same base element, the directly emitted waves and reflected waves overlap destructively. Accordingly, a greater wave amplitude is radiated in the direction pointing from the reflection fingers to the pair of fingers of one and the same base element than in the opposite direction and this is therefore designated as the forward direction.
Because of this characteristic, interdigital transducers which radiate in essentially only one direction can be constructed on the principle of the single-phase unidirectional transducer so as to prevent losses due to wave radiation in unused directions. Therefore, the principle is suitable for producing components with low insertion loss. However, it must be ensured that echoes due to reflections at the transducers as a whole do not interfere with the transmission behavior. In constructions [1] and [2] referred to above, the reflection of every transducer is suppressed. Apart from suitable selection of matching networks, this is accomplished primarily by the adjustment of spatially-dependent reflection factors of the reflection fingers from one base element to next base element. In construction [3] above, however, not only are the echoes resulting from reflections at the transducers not suppressed, they are even utilized for structuring the transmission behavior. Accordingly, filters with a small form factor (ratio of 30-dB bandwidth to 3-dB bandwidth), that is, with steep edges, can be realized without the very long layouts which are otherwise necessary.
However, construction [3] has the disadvantage that frequency dependencies of insertion loss and/or group delay time which are asymmetric relative to the center frequency, for example, sloping passbands, cannot be realized in a predetermined manner.